Generate safe sourcemaps

The previous comment syntax was unsafe with Internet Explorer. This
change copies the equivalent upstream change to use the more robust
nested comment:

```javascript
/*
//@ sourceMappingURL=file.min.js.map
*/
```

See:
    https://github.com/mishoo/UglifyJS2/issues/108
    http://bugs.jquery.com/ticket/13274
Update test fixtures
Merge pull request #44 from acdha/safe-sourcemaps

Generate safe sourcemaps